A study was planned to see if the mammillary bodies have any influence upon orienting response Thirty-nine male rats were divided into two groups. One group of animals received electrolytic lesion of the mammillary bodies through stereotaxically implanted electrodes (mammillary body group), and the other group received the same treatment short of electrolytic lesion(operated control group). All animals were tested for orienting response 3 weeks after surgery. Orienting response was regarded as elicited when an animal in alertness ceased ongoing activity upon 1,000 Hz tone presentation and/or turned head toward the source of stimulus. Weak (momentary and inconspicuous) and strong (marked and more persistent) responses were discriminated. Occurrence and duration of orienting response were measured, and the rate of habituation was estimated in 20 trials. Shift of attention from sound to light stimulus (10 trials) as well as orienting response of thirsty animals to sound stimulus while drinking (5 trials) and while at rest (5 trials) were also tested. 1. Simple orienting respons to sound stimulus tended to occur more often and its duration tended to be shorter in the mammillary body group than in the operated control group. 2. With repetition of trials, the occurrence of orienting response in each trial decreased progressively in both groups (habituation). However, the rate of habituation was significantly less prominent in the mammillary body group than in the operated control group. 3. Under attention¡¤shift situation, orienting response tended to occur less often and its duration tended to be shorter in the mammillary body group than in the operated control group. 4. Orienting response to sound stimulus while drinking to quench thirst tended to occur more often and its duration tended to be shorter in the mammillary body group than in the operated control group. In quiet state, however, there existed no group difference with regard to the occurrence and duration of the orienting response. Although no obvious inference can be drawn from the above results, they may suggest the mammillary bodies being involved in the orienting response as a control mechanism inhibitory to the occurrence and facilitatory to the habituation of the response.